The present invention relates to a process for forming solder lands in a printed wiring board manufacturing method.
In the case of loading and soldering electrical components onto a printed wiring board having a wiring circuit of a required pattern provided on an electrical insulating sheet, a solder resist process is carried out in order to prevent the solder from being adhered to an unnecessary portion of the wiring circuit, save and batch the solder, prevent short circuit caused by the solder bridge, form suitable solder fillet, and protect the lead wire of the wiring circuit from the external environments.
As a solder resist process a screen printing process and a photographic developing process are adopted. Generally, the screen printing process is apt to adopt in a non-through hole wiring board and the photographic developing process is apt to adopt in a through hole wiring board (including a multilayer printed wiring board, since the through hole wiring board has more fine circuits than that of non-through hole wiring board and thus a pattern of solder resist becomes fine so that it is obliged to adopt the photographic developing process which is advantageous to the fine solder resist pattern.
The photographic developing process is, however, deficient in mass production and becomes high in cost due to high source material so that a non-through hole wiring board which requires mass production and low cost, can not be adopted.
The solder resist by the screen printing process is carried out as shown in FIG. 2, after an etching step in the subtractive process (the etched foil process). That is, an etching resist 3 corresponding to a required pattern is applied (FIG. 2b) on a copper foil 2 of a copper clad laminate 1 which is subjected to a cutting and machining process shown in FIG. 2a, thereby forming an wiring circuit 5 of the required pattern on the electrical insulating sheet 4 by the required etching process (FIG. 2c).
A solder resist 6 shown in FIG. 2d is then, printed on the wiring circuit 5 shown in FIG. 2c through a required solder mask.
Since a conventional process for forming solder resists 6 has a limitation in precise formation of the solder resist followed to the fine circuit, in the case of performing the processing step shown in FIG. 2d particularly, in the case of forming solder lands 7 of solder resists 6 as shown in FIGS. 3a and 3b, the solder land 7 is printed in accordance with the required solder mask diameter thereof in a first printing step, and then, a solder land 8 having a diameter slightly larger than the above solder mask diameter (for example, 0.1 mm to approximately 0.2 mm in diameter is printed in a second printing step, thereby finishing the whole surface thereof. Alternately, as shown in FIG. 4a, solder resist printing is carried out in a first printing step and a hemming or trimming 9 of a solder land portion of the solder resist 6 is carried out by character ink in a second printing step as shown in FIG. 4b. The printing steps shown in FIGS. 4a and 4b may be reversed.
In the process for forming solder resist in the printed wiring board manufacturing method, the following three requirements in printing technique are present.
(1) The copper foil portion of the wiring circuit to be coated by the solder resist should not be exposed in copper foil itself.
(2) In the solder land, a blur of the solder resist should be supressed and the area of the solder land should be secured in accordance with designed dimension.
(3) The film coated pressure of the solder resist must be guaranteed even with a lowest level.
In the case of performing the above requirements completely, however the above requirements (1) and (2) are mutually contradictory. This is a cause of limitation in fine solder land formation by the screen printing process.
In order to attain the first requirement, that is, as shown in FIG. 5a, during the manufacture step of printed wiring board and after etching, the presence of wiring circuit 5 must be taken into account that the printed surface of the solder resist 6 becomes uneven. As shown in FIG. 5b, in the case of printing the solder resist 6, when the solder resist 6 is printed onto the wiring circuit 5 by a squeegee 11 while using a screen 10, it is required that a printing pressure is applied to the squeegee 11, thereby flowing solder resist ink into recess portion 12.
In this case, in principle, the higher the printing pressure, the more the solder resist ink fills into the recess portion 12 and the lower the viscosity of the ink, the more the solder resist ink flows into the recess portion 12.
In order to suppress the blur of solder resist as the second requirement, then, the printing pressure of the squeegee 11 must be decreased as soon as possible.
As shown in FIG. 6a, that is, when the printing pressure of the squeege 11 is high, a tip portion 11a thereof is bent and the contact area of a printed surface 13 of the printed matter 14 and the off bent tip portion 11a of the squeegee 11 becomes large (see FIG. 6b), so that the snapping-off of a form plate becomes worse and thus the blur of solder resist is caused.
It is found from the silk screen printing that the printed-out portion of a screen 10 must be separated from the printed surface 13 as soon as possible, but in the case of large contact area of tip portion 11a of the squeegee 11, the screen 10 is separated from the printed surface 13 for a long time, and thus the snap-off of the form plate becomes worse.
This phenomenon is the same as the following phenomenon. When a stamp is used, if the stamp does not press its surface onto a paper at one time, but press the surface to the paper in the order, its front portion, its center portion and its back portion, and then separate its surface from the paper in the order, the front, the center and the back portion, stamping without blur may be obtained, while if the whole surface of the stamp is pressed onto the paper at any time, and it is subjected to a slight horizontal vibration (slight vibration accompanied with squeegee movement),the stamping with blur may be obtained.
Then, the lower printing pressure of the squeegee 11, the more suitable printing with supressed blur can be obtained, and thus the higher printing precision with high ink 11 viscosity can be obtained.
FIGS. 6c and 6d show a relation between the squeegee and the printed surface 13 in the case of low printing pressure of the squeegee 11.
As is found from the above descriptions, the condition in screen printing technique for the first requirement, that is, the complete coating of the copper foil portion required in the process of forming solder resist by the screen printing method, is as follows. It is preferable to make the printing pressure high and to make the viscosity of solder resist ink low. On the contrary, the condition in the screen printing technique for the second requirement, that is, the suppresion of the blur of the solder resist is as follows. It is preferable to make the printing pressure of the squeegee 11 low and to make the viscosity of solder resist ink high. One could not avoid performing the present screen printing method under the above inconsistent conditions at only 1.about.2 sec during which the squeegee 11 is moved.
Even though processes shown in FIGS. 3 and 4 are adopted in the case of following solder resist, fundamental unevenness in surface condition of the printed surface is not entirely changed and thus the presence of the above inconsistent relationship causes a technical limitation in mass production of the solder 7 having a diameter up to 1.4 mm in diameter in case of forming solder resist 6.
In this present guarantee in thickness of resist coating film described as the third technical requirement in the case of forming solder resist by the screen printing process, as shown in FIG. 7, the film thickness of coated solder resist 6 provided on the wiring circuit 5 comprises a portion A and a portion B, and the thickness of the portion B is about a half of that of the portion A, so that problem of moisture vapor resistance for the portion B arises in environmental test, and thus the guarantee in thickness of the resist film is also required for the portion B. This has an important problem for the leveling of the whole film thickness.